2.
Main Vectors
2.1 Introduction
The scope of this chapter is to provide an overview of the main vectors of
medical importance encountered in refugee/displaced person camps.
Three types of vectors are discussed; arthropods, snails and rodents. These
are the major causes of disease in many tropical and subtropical countries.
They can result in a high rate of morbidity and mortality, particularly in
temporary settlements.
2.2 Some definitions
2.2.1 Vector
A vector may be any arthropod (insect or arachnid, see Table 2.1.) or animal
which carries and transmits infectious pathogens directly or indirectly from
an infected animal to a human or from an infected human to another human.
This can occur via biting (e.g. mosquitoes, tsetse flies), penetration (e.g.
guinea worm), or the gastrointestinal tract (e.g. contaminated food or drink).
2.2.2 Biological vector
The pathogens (parasites or arboviruses) in the infested host, are ingested by
the vector where they undergo change and multiplication in order to mature
to an infective stage. This usually takes several days before they are capable
of being transmitted to a new host (e.g. human malaria parasite).
2.2.3 Mechanical vector
Mechanical vectors transmit diseases by transporting the causative agent
from contaminated material (e.g. faeces) on their feet or mouth parts and
then spreading the pathogens or parasites on to human food, drink, faces or
eyes.
4
MAIN VECTORS
2.2.4 Arthropods
Arthropods are small animals with jointed legs, which include insects (class
Insecta − e.g. mosquitoes, flies), arachnids (class Arachnida − e.g. ticks, and
mites), crustaceans (e.g. Cyclops, guinea worms), and centipedes/millipedes;
all of which differ in respect of their antennae, wings, or legs.
Table 2.1. Classification of the main arthropods of medical importance
ARTHROPODS
INSECT
ARACHNIDS
Class Insecta
Class Arachnida
Identification
Identification
1. Three distinct body regions (head, thorax,
abdomen)
1. Two distinct body regions (cephalothorax,
abdomen)
2. Three pairs of legs
2. Four pairs of legs (except larval mites,
which have three pairs)
3. Often have wings
4. One pair of antennae
3. Never have wings
5. Segmented abdomen
4. No antennae
Order Diptera
n Mosquitoes
Order Acarina
n Mites
n
Flies
Order Heteroptera
n Bugs
5. Abdomen usually not segmented
n
Ticks
Order Araneida
n Spiders
Order Anoplura
n Lice
Order Siphonaptera
n Fleas
Order Scorpionida
n Scorpions
Order Dictyoptera
n Cockroaches
Source: Sabatinelli, 1996
5
EMERGENCY VECTOR CONTROL
2.3 Main vectors often involved in vector-borne disease
epidemics
2.3.1 Mosquitoes
Mosquitoes are a large arthropod group with 3,100 species occurring in the
world. Only about a hundred of them are vectors of human disease. Mosquitoes can be divided into two subfamily groups;
1. The anopheline subfamily including the most important mosquito genus
Anopheles which is responsible for transmitting malaria. Anopheles are
also involved in transmission of filariasis in West Africa.
2. The culicine subfamily where the important genera Aedes, Culex, and
Mansonia belong. Several diseases are transmitted by them such as yellow
fever, and dengue by Aedes, encephalitis virus by Culex. All of these
mosquitoes are also involved in the transmission of filariasis (Table 2.3).
Both male and female mosquitoes feed on sugary secretions such as nectar
from plants. In all species, only the female mosquito takes blood-meals from
animals and/or humans. The female mosquitoes are attracted by the odour,
the carbon dioxide and the heat from animals and humans. The blood sucked
is used to provide proteins to mature batches of eggs.
The life cycle of the mosquito consists of four stages (Table 2.2.): the immature stages of egg, larva, and pupa require an aquatic environment. The
adult develops in aerial and terrestrial environments.
The females are able to lay between 30 and 300 eggs at a time, according to
species. The anopheline mosquitoes lay their eggs separately over the surface
of any kind of unpolluted water. The culicine mosquitoes, Culex and Mansonia, lay their eggs on water as an egg-raft form. The eggs of Aedes mosquitoes are laid just above the water line or in wet mud. Provided that they
are kept dry they can survive for 3 to 4 years and hatch only when flooded by
rising water levels or heavy rain (OMS, 1973).
6
MAIN VECTORS
Table 2.2. Characteristics of the different life stages of the mosquito
SUBFAMILIES
Anopheline
Genus Anopheles
Culicine
Genus Culex
Genus Aedes
Genus Mansonia
Eggs
float
do not float
raft of 25 - 100
eggs
Larval stage
Pupal stage
Adult stage
Long palps
Short palps
Source: Adapted from Cheesbrough, 1987
7
Aquatic plant
EMERGENCY VECTOR CONTROL
Table 2.3. Behaviour of mosquitoes and diseases they transmit
Genus
Anopheline
mosquitoes
Anophelines breed in
non polluted water
Biting period : NIGHT
Breeding site
Place
found
Disease and distribution
edges of rivers,
swamps,
impoundments,
ditches, tanks,
saltwater habitats
protected from wave
action, rice fields,
temporary rainpools,
hoofprints.
Worldwide
n
n
Aedes spp. are primarily
forest mosquitoes.
Tin cans, plastics, car Worldwide
tyres, gutters,
ornamental ponds,
tanks, jars, any type
of container, waste
disposal areas, tree
holes.
Brugian filariasis: Asia
n
O’nyong nyong virus: Africa
n
n
n
n
Biting period : DAY
Bancroftian filariasis: Asia
and Africa
n
Aedes mosquitoes
One species lives in
close association with
man, in any kind of
human settlement. The
A. aegypti breeds in any
small water collection.
Malaria: Tropical and subtropical areas
n
Yellow fever: Africa and
Americas
Dengue: Africa, Americas,
Asia
Dengue Haemorrhagic
fever: Americas, Asia
Bancroftian filariasis:
Pacific
Other arbovirus: Africa,
Americas, Asia
Culex mosquitoes
C. quinquefasciatus
breed in any dirty water
in urban and rural areas.
Other species are also
very common in rice
fields in Asia.
C. quinquefasciatus
Worldwide
Waste water ditches,
latrines, septic pits,
cesspools, drains,
waste disposal.
n
n
Bancroftian filariasis: Most
tropical areas
Encephalitis virus: Africa,
Americas, Asia, Europe
Biting period : NIGHT
Mansonia mosquitoes
Mainly associated with
aquatic plants, in rural
areas where irrigation
canals occur.
Ditches, ponds,
irrigation canals,
swamps.
Essentially
tropical
n
Worldwide
n
Brugian filariasis: Asia
Other arbovirus: Rare in
Africa and Americas
Biting period : NIGHT
It is a vicious biter
Sources: Adapted from Birley, 1991; Thomson, 1995; Sabatinelli, 1996; Chavasse and Yap, 1997;
Rozendaal, 1997
8
MAIN VECTORS
At tropical temperatures the larval period is 4-7 days, after which time it
become a pupa. The larvae feed on bacteria and small aquatic organisms. All
larvae need to come to the surface to breathe, except for the larvae of Mansonia mosquitoes which extract oxygen from the roots of certain aquatic
plants.
The pupal stage lasts 1-3 days in warm areas. The pupa does not feed, it
breathes air and turns or twists its body with short writhing movements.
Rapid metamorphosis occurs inside the pupa.
Male adults which are recognisable by their hairy antennae, generally emerge
first and wait to fertilize the emerging female mosquitoes.
2.3.2 Non-biting flies
Domestic flies, including the housefly are probably the most widespread
insects in the world and certainly the one most closely associated with man.
Like mosquitoes they belong to the order Diptera. All of these species,
Musca domestica (Housefly), Musca sorbens (Facefly), and Chrysomya spp.
(Blowfly) are considered to be of medical importance because they transmit
diseases by transporting pathogens between people or from faeces to food
causing diarrhoeal diseases and trachoma.
The life cycle of the fly consists of four stages : egg, maggot (larva), pupa
and adult. The female lays her eggs in moist, organic material. These eggs
hatch after 8 to 48 hours and become maggots which thrive on organic matter, animal or human excrement. After a few days to several weeks, depending on the temperature, quantity of food, and the species, the maggot moves
underground where it becomes a pupa. It takes 2 to 10 days for the maggot to
be transformed into an adult within the pupa. The female rarely lays more
than 100 to 150 eggs in each batch, 4 to 5 of which are produced during her
lifetime (Busvine, 1980). Adult flies have a lifespan of one to two months
depending upon the species and life conditions.
Musca domestica
The housefly, Musca domestica, tends to breed in animal excrement in rural
areas. In urban areas they breed in organic domestic waste such as vegetable
matter. They land on both faeces and human food, and feed on both. Contrary to popular belief, these flies do not lay their eggs in latrines (Chavasse,
1997). The high density of flies associated with a crowded human popula-
9
EMERGENCY VECTOR CONTROL
tion, in unsanitary conditions such as refugee
camps, increases the risk of transmission of
disease and can cause epidemics.
M. domestica is about 6 to 7 mm long and has a
wing span of 13 to 15 mm. It is recognisable
due to its greyish colour, and 4 dark longitudinal stripes along the back of the thorax.
Figure 2.1.
M. domestica, (WHO, 1997)
As a mechanical vector this species of fly may
be responsible for carrying pathogens which
cause diseases such as infantile diarrhoea, shigellosis diarrhoea, dysentery, typhoid, and also
intestinal worm eggs. It has been proved in
Pakistan (Chavasse, 1998) and in the Gambia
(Emerson, 1999) that fly control reduced incidence of diarrhoea by about 24 %.
Musca sorbens
M. sorbens or facefly also acts as a mechanical
vector in the transmission of diseases. It has a
worldwide distribution in the tropics. These
flies breed, amongst other things, in human
faeces around settlements. They are also known
to feed on the secretion produced by eyes of
people, especially children. They do not breed
in latrines. The density of these flies may be
very high in unsanitary conditions (Chavasse,
1997).
Figure 2.3.
M. sorbens (WHO, 1997)
M. sorbens is 6 mm long and has a wing span of
around 15 mm. It is grey and has 2 dark stripes
on the back side of the thorax.
M. sorbens is responsible for carrying diarrhoeal diseases but it is often
implicated in the transmission of the eye disease trachoma, and it has recently been shown in the Gambia that fly control reduced incidence of trachoma by 70 % (Emerson, 1999).
10
MAIN VECTORS
Chrysomya spp.
Figure 2.4.
Chrysomya spp. (UNHCR,
1996)
Chrysomya spp. or blowfly is distributed
worldwide. It has a strong preference for
breeding in all kinds of open latrines, decomposing meat or fish, garbage, and animal excrement. In refugee camps the simple pit
latrines often have no lids on the defaecation
holes, offering an ideal breeding site for these
flies. Blowflies do not enter houses, but are very
active in market places. The density of the
blowfly population may rise as the number of
simple pit latrines in a camp increases
(Chavasse, 1997).
The blowfly is robust and measures 10 mm in length with a compact body.
Its colour varies between a shiny blue and a shiny green.
Chrysomya spp. act as a mechanical vector and may be responsible for carrying pathogens causing diarrhoeal diseases such as dysentery, and also
intestinal worm eggs. If pit latrines are humming with blowflies and crawling
with their maggots this will probably be a deterrent for using them.
2.3.3 Lice
Three species/subspecies of human louse occur in the world. They are all,
male and female, blood-sucking ectoparasites. The body louse (Pediculus
humanus humanus), the head louse (Pediculus capitis) and the pubic or socalled crab louse (Pthitrus pubis) have approximately the same biology.
Three stages constitute their life cycle; egg, nymph, and adult. Only the body
louse is a vector of disease (typhus). The others do not transmit disease but
may cause irritation and severe itching. Lice are spread by close contact
between humans.
Pediculus humanus humanus (Body louse)
The body louse is found attached to clothing in close contact with the skin.
The females lay and glue their eggs (also called nits) at a rate of about 10
eggs a day onto fibres of clothing, especially on woollen material. They
never attach their eggs to human body hairs (Busvine, 1980). Eggs hatch
11
EMERGENCY VECTOR CONTROL
after one week. The nymphal stage lasts from 8
to 9 days. The adult stage lasts up to 10 days
during which many blood meals are taken.
The adult body louse is between 3 to 4.5 mm
long. It is wingless and has a flattened body.
Figure 2.5.
Pediculus humanus humanus
(UNHCR, 1996)
The body louse is responsible for the transmission of typhus, not directly by biting, but by
causing itching in the human host. Scratching
favours the penetration of the contaminated
faeces of the louse or the contaminated louse
itself into the skin, which may cause epidemic
typhus. It can also carry relapsing fever and
trench fever.
2.3.4 Mites
Mites belong to the order Acarina (Class Arachnida). There are over a thousand species which are parasitic on mammals, birds, and some species on
humans. Two species of human mite are of medical importance; the biting
mites called "chiggers" (Trombiculid mites), and scabies mites (Sarcoptes
scabeie). Scabies mites are common in human settlements such as refugee
camps.
Mites have eight legs and a body with little or no segmentation. They are
very small, 0.5 mm to 2 mm in length. In most species, their life cycle consists of four stages; egg, larva, nymph and adult. Transmission of scabies
mites between humans occurs by direct contact.
Biting mites (trombiculid mites)
After emerging from the eggs the larvae crawl onto vegetation or into woodlands seeking an animal or human host. They attack and feed on the skin of
reptiles, mammals, birds and humans. On human they seek out areas where
clothing is tight against the skin. The larvae feed on the host only once, then
between two days and one month later the larvae drop to the ground and
enter the soil to develop into nymphal and adult stages (Rozendaal, 1997).
The nymph and the adult never feed on animals or humans. They feed on
small insects and their eggs or on other mites.
12
MAIN VECTORS
These mites transmit rural typhus in Asia and Pacific, known as scrub typhus.
Scabies mites
Figure 2.6.
Scabies mite (UNHCR, 1996)
Almost the complete life of the scabies mite is
spent on and in the human skin where they
feed, procreate and lay their eggs. The adult
Sarcoptes scabeie burrow winding tunnels of
1.5 mm in length in the surface of the skin.
Scabies mites are found everywhere on the
body. Scabies disease results from an allergicreaction to the infection of the skin caused by
the burrowing of the mites.
2.3.5 Fleas
The fleas, both male and female are blood-feeding and belong to the order
Siphonaptera. Around 3,000 species occur worldwide, but only a dozen
species take a blood-meal on humans. Rat fleas (Xenopsylla cheopis) and
human fleas (Pulex irritans) are the most important from a medical viewpoint. They jump from the ground to the lower parts of human legs, but can
bite them anywhere on the body. Fleas are able to survive more than one
month without having a meal and their lifespan may reach 17 months while
taking regular blood meals (Sabatinelli, 1997).
Figure 2.7.
A female flea (WHO, 1997)
Four stages constitute the life cycle of fleas;
egg, larva, cocoon (where it pupates), and adult.
Eggs are dropped on the ground near to the
sleeping place of the host. They hatch after a
period of 2 to 14 days. Eggs produce larvae
which live among debris, dirt and dust, organic
debris provides the main food. The larva spins a
silken cocoon over a two to three week period
and then pupates for one to two weeks before
becoming an adult (Service, 1986).
The adults vary in size from 1 mm to 4 mm. The body is compressed and
flattened laterally. Black or dark brown are the main colours. Fleas do not
develop wings but they have powerful hind legs to propel them through
jumps of 20 cm vertically and 30 cm horizontally.
13
EMERGENCY VECTOR CONTROL
Fleas can act as vehicles for parasitic tapeworms (in humans). They cause
irritation and sometimes allergies and they are also involved in bubonic
plague and murine typhus.
2.3.6 Rodents
Rats are located in almost all human communities. Three species of medical
importance have been found and may present a danger to the human population. They belong to the family Muridae, and are Rattus norvegicus, Rattus
rattus and Mus musculus (Table 2.4.). All are very close to man, and found
in rural and urban areas.
Table 2.4. The three main rodents of medical importance
Species
Rattus norvegicus
Rattus rattus
Mus musculus
Other name
Brown rat or Norway
rat
Roof rat or Black rat
House mouse
Weight
500 g
250 g
20 g
Length
45 cm
40 cm
18 cm
Habitat
Principally in sewers
and holes and feeds
on garbage.
Under the roof of any
type of building.
Around supplies of
grain, cereals, and
flour.
Source: Sabatinelli, 1996
Rats feed on any kind of food. They cause destruction of foodstuffs by contamination through urine, faeces, or by simple contact. This can result in
important financial and nutritional losses to the population, especially in
emergency situations when essential supplies are already critical (Pan
American Health Organization, 1982).
Female rats give birth to 5-8 young which become adult and reach sexual
maturity in two months.
14
MAIN VECTORS
Roof Rat - Rattus rattus
Norway Rat - Rattus norvegicus
Figure 2.8. Morphological differences between rats
Centre for Disease Control, US Public Health Services, Atlanta
Rats may transmit diseases such as Lassa and leptospirosis to humans
through contact with faeces, urine, and nasal or oral secretions of infected
rats. They also act as reservoirs for pathogens which cause Murine typhus,
and plague (through fleas), lyme disease, and relapsing fever (through ticks)
(Chavasse and Yap, 1997).
2.3.7 Other vectors
The vectors described in Table 2.5 are of medical importance in certain
areas. If a camp is located where these arthropods exist they may cause
problems depending upon the sanitary conditions of the affected people.
15
EMERGENCY VECTOR CONTROL
Table 2.5. Vector description and main diseases they transmit
Vector (genus)
Particularity
Breeding sites and habits
Tabanid or
Horsefly
They are very
robust
1. Chrysops
Length;
6 to 10mm
Only the female feeds on
any animal. She lays 100 to
1000 eggs according to
species. They breed in moist
and wet ground.
Tsetse fly
Glossina
Mortisans group
(savannah flies)
Very long
proboscis
Palpalis group
(river bank flies)
9 to 25mm
in length
Sandfly
Less than
3mm long
Phlebotominae
wide wings
Very long legs
Bedbug
7mm long
Cimex spp.
Reduviid bugs
brownish
insects
flat and oval
body
Blackfly
Simuliidae
Small insect,
1 to 6mm in
length
Both males and females
suck blood. Tsetse flies are
viviparous. They deposit
their larvae in damp ground
and arid areas.
The females only are bloodsucking at night. They are
located in the tropics and
subtropical areas south of
Europe. They breed in moist
and wet ground.
They lives in temperate and
tropical zones. They are
active only at night where
they feed on humans and
animals.
They bite during the day.
Only the female sucks the
blood of animals and
humans. They breed and live
in all kinds of unpolluted
water, vectors in Africa
breed only in fast flowing
oxygenated streams or
rivers. They have a
worldwide distribution.
16
Disease and distribution
n
n
n
n
n
n
Loa loa filariasis:
West and Central
Africa
Sleeping sickness:
Africa
Cutaneous and
visceral (Kala Azar)
Leishmaniasis occur in
Sudan, Latin
Americas, India, Asia,
Middle East, and
Southern Europe
Bedbugs cause
nuisance such as
itchiness: Worldwide
Chagas disease is
transmitted by
triatomine bugs in
South and Central
Americas, and in
some parts of
Caribbean
Onchocerciasis or river
blindness: Africa, and
some parts of Latin
America
MAIN VECTORS
Vector (genus)
Particularity
Breeding sites and habits
Cockroach
5 to 73mm
in length
These insect are very agile
and fast and live in colonies.
They have a preference for
man-made structures where
it is warm. In the tropics
they may live and breed
outdoors. Latrines may be
infested in a refugee camp.
They are particularly active
at night.
two pairs of
wings
flattened
appearance
Yellow-brown
to dark colour
Tick
7 to 20mm
in length
Hard tick
Hard back
Soft tick
Soft back
Cyclops
(Cyclopidae family)
Small
crustacean
0.5 to 2mm
in length
Water snail
(mollusc)
1. Biomphalaria
2. Bulinus
3. Oncomelania
Aquatic snails
Both males and females
feed on warm-blooded
animals and humans. They
are attracted by the carbon
dioxide from their prey. Hard
ticks are located in
vegetation and soft ticks live
in close association with
available prey. They can
survive several years of
starvation.
These are the intermediate
host of the guinea worm.
They live in any artificial or
natural accumulation of
stagnant water which may
be used as drinking water.
Snails are found in all
suitable types of water
except for salty and acidic
waters. Snails serve as an
intermediate host of
shistosomiasis worms.
Disease and distribution
n
n
n
n
Cockroaches act as
mechanical vectors
and may transmit
diarrhoeal diseases,
typhoid fever,
dysentery, viral
diseases:
Worldwide
Relapsing fever:
Worldwide
Q-fever: Africa,
Americas
Lyme disease
Arbovirus diseases:
Worldwide
Ticks are very painful
biters and can cause
serious loss of blood
n
n
Guinea worm or
Dracunculiasis: Africa
Shistosomiasis (or
bilharzia): In the
tropics, mainly in
Africa and East Asia
Source: Adapted from Birley, 1991; Thomson, 1995; Sabatinelli, 1996; Chavasse and Yap, 1997;
Rozendaal, 1997
17